The invention relates to a method of operating a displacement device for the throttle valve (1) of an internal combustion engine of a motor vehicle, wherein the throttle valve (1) can be displaced in an opening direction by a gas pedal (2) and, alternatively, by an electromotive setting drive (3) against the force of a return spring (4). A first potentiometer (5) is provided to produce a signal which represents the position of the throttle valve, a second potentiometer (6) is provided to produce a signal representing the position of the setting drive (3), and an electronic control device (7) is provided to process these and other signals and to control the setting drive.
Such displacement devices are used for automatic idling control and also for automatically maintaining a predetermined driving speed. In both cases, when the gas pedal is completely released the throttle valve is displaced by the electronic control device and the electromotive setting device as a function of the instantaneous need for idling power or the difference between actual and predetermined speed of the vehicle.
If such displacement devices are used only for regulating idling, the setting range of the electromotive setting drive can be limited in opening direction in simple manner by means of a mechanical stop. In this way, assurance is had that, in case of a fault in the setting drive, the corresponding potentiometer, or the electronic control device, the throttle valve can be opened electromotively with maximum opening limited by this stop. If the displacement devices, however, are used for the automatic maintenance of a predetermined speed, the setting range of the electromotive setting drive must extend over the entire setting range of the throttle valve from a position of minimum idling powers LL.sub.min via a position for maximum idling power LL.sub.max up into the full-load position VL. In case of a defect, therefore, unintended displacement of the throttle valve up into regions which are important from a safety standpoint is possible.
Various proposals have already been made for the avoidance of such risks. Thus, for instance, safety contacts can be provided which are associated with given positions of gas pedal, of the electromotive setting drive or of the throttle valve and which, either alone or in combination with signal values of the potentiometers, permit plausibility tests. These tests, upon detection of a defect, produce the disconnecting of the electromotive setting drive, including at least a displacement of the throttle valve in the direction of closing (see Europe 0369 061 B1 and German 39 00 437 C1).
Since the safety contacts themselves, however, represent a new source of error and since they are relatively expensive due to the high quality requirements, and can be adjusted only at considerable expense, it is desired to solve the safety problems in a manner which is more favorable in price and simpler in construction.